Could a recent popular strategy to unionize workers be grinding to a halt? A Supreme Court hearing on Nov. 13 could reshape the modern labor landscape, potentially turning illegal a common labor tactic that has led to the unionization of hundreds of thousands of workers.

The strategy involves having a company sign a “neutrality agreement,” in which an employer agrees not to oppose any unionization efforts by its workforce. Major labor organizations, such as the Service Employees International Union and the Unite Here hotel workers union according to The New York Times, are able to take these agreements and more easily talk with employees, sometimes even on company property.

In Unite Here Local 355 v. Mulhall, however, an employee from Mardi Gras Gaming in Florida alleged that Unite Here’s neutrality agreement with his employer was illegal. The 11th Circuit agreed with the employee, Martin Mulhall, saying that the agreement was a “thing of value” given from employers to unions, which is illegal under federal labor law. This finding went against former rulings by the 3rd and 4th Circuits, which found that neutrality agreements were not things of value.

Benjamin Sachs, a professor of labor law at Harvard Law School, told the Times that this case should be “the most significant labor case in a generation.” According to Sachs, if the Supreme Court rules against Unite Here, one of the biggest tools in the private sector’s arsenal to unionize workers will disappear. The Times says that by some estimates, over half of recent successful unionization attempts have come via neutrality agreements.

Sachs says that this case, along with Harris v. Quinn, could provide precedent to many future labor rulings. In Harris, a 2003 executive order from then-Illinois Governor Rod Blagojevich ruled that home-care workers in the state were public employees, which allowed them to unionize. Workers who joined the Service Employees union paid dues, but those who did not were forced to pay “fair share” fees to help finance the union. One worker, Pamela Harris, disputed the fees, but the 7th Circuit ruled they were fair under the workers’ collective bargaining agreement, citing 1990’s Abood v. Detroit Board of Education.

These cases aren’t the only labor stories we’re watching. For more labor/employment, check out these InsideCounsel articles: